Blender 2.8 How to edit via the Python API an action and change keyframes
The related answers (provided by @batFINGER) answered my question, but I provide here an example for completeness sake.

# Configuration
action_name = 'MyAction'
data_path = 'location'
index = 2                   # Z axis

# Find the appropriate action
action = bpy.data.actions.get(action_name)
if action:
    # From this action, retrieve the appropriate F-Curve
    fcurve = action.fcurves.find(data_path, index)
    if fcurve:
        # Iterate over all keyframes
        for kfp in fcurve.keyframe_points:
            # Print current keyframe info
            print('Frame = {:04}; Value = {}'.format(kfp.co[0], kfp.co[1]))
            # Change keyframe data
            # Push back in time by 2 frames
            kfp.co[0] += 2
            # Move F-Curve up by 1
            kpf.co[1] += 1